A low-sensitization stainless steel and a preparation method and application thereof

Abstract

The application belongs to the technical field of medical stainless steel preparation, and relates to a low-sensitization stainless steel and a preparation method and application thereof. The preparation method of the low-sensitization stainless steel comprises the following steps: (1) preparing ferrite stainless steel billets by a vacuum induction melting method; (2) sequentially performing hot rolling and cold rolling on the billets to obtain rolled materials; (3) polishing and degreasing the surface of the rolled materials, and then placing the rolled materials into a sealed atmosphere furnace to perform high-temperature nitriding; (4) after the nitriding and heat preservation are completed, quenching the rolled materials by immersing the rolled materials into ice water; and (5) removing the oxide skin on the surface of the rolled materials to obtain the low-sensitization stainless steel. The low-sensitization stainless steel prepared by the application is superior to 316L stainless steel in tensile strength and work hardening performance, has excellent corrosion resistance and cell biocompatibility, and is suitable for batch preparation of various low-sensitization medical small metal components such as orthodontic correction wires and orthopedic tension band steel wires.

Description

Technical Field

[0001] This invention belongs to the field of medical stainless steel preparation technology, and relates to a low-allergenic stainless steel, its preparation method and application, and particularly to a preparation method for nickel-free stainless steel by solid nitriding in-situ austenitization of Fe-Cr-Mo ferritic stainless steel. Background Technology

[0002] Medical-grade austenitic stainless steel is widely used in the manufacture of various implantable and surgical instruments due to its excellent comprehensive mechanical properties and corrosion resistance. Among them, AISI 316L stainless steel (containing approximately 10–14 wt% Ni) is one of the most widely used medical-grade stainless steel materials in clinical applications.

[0003] However, nickel is a common allergenic metallic element. Studies have shown that after prolonged contact with human tissue, nickel ions from nickel-containing medical stainless steel can gradually dissolve and induce allergic reactions, manifesting as local inflammation, tissue necrosis, and other clinical symptoms, posing a potential risk to patients' health. Therefore, developing low-allergenic or even nickel-free medical stainless steel materials has become an important research direction in this field.

[0004] To replace nickel-containing austenitic stainless steel, existing technologies have proposed using elements such as Mn and N to replace Ni in order to stabilize the austenitic structure. For example, 200 series austenitic stainless steel, which uses Mn-N to replace Ni, has achieved nickel-free production to a certain extent. However, 200 series stainless steel has the following problems: (1) It has a large work hardening coefficient, making it difficult to form plates and fine wires from cast ingots; (2) It has a large springback during the forming process of parts, which makes it difficult to meet the industrial precision requirements for miniaturized and micro-sized medical implants with strict dimensional tolerance requirements.

[0005] On the other hand, Fe-Cr-Mo ferritic stainless steels (such as the Fe-18Cr-3Mo series) exhibit superior resistance to chloride pitting and crevice corrosion compared to 316L, and are characterized by easy machining, low strain hardening, and ease of forming. However, the insufficient strength and wear resistance of this series of ferritic stainless steels limit their application in load-bearing medical implants. Furthermore, damaged cells and postoperative inflammation sites produce organic acids such as lactic acid and citric acid, causing the local pH value to drop to 4.5–6.0, placing higher demands on the acid corrosion resistance of stainless steel—once corrosion occurs, dissolved chromium and other metal ions may also trigger allergic and inflammatory reactions.

[0006] Nitrogen is a strong austenite-forming element. Injecting nitrogen into stainless steel can simultaneously improve the material's strength and corrosion resistance without significantly reducing its plasticity. Existing technologies have reported improvements to the surface properties of stainless steel through nitriding. However, conventional nitriding processes (such as gas nitriding and ion nitriding) typically suffer from thin nitrided layers, long processing times leading to grain coarsening, and decreased material plasticity after treatment, making it difficult to meet the requirements of uniform performance and precise dimensions across the entire cross-section for small medical components. Furthermore, a mature and feasible industrial solution is currently lacking for achieving full-section austenitization on a Fe-Cr-Mo ferritic stainless steel matrix through nitriding while simultaneously avoiding grain coarsening and the precipitation of brittle nitrides.

[0007] In summary, the existing technology lacks a complete manufacturing process that can simultaneously achieve nickel-free properties, good processability, excellent corrosion resistance and biocompatibility, and is suitable for mass production of small medical stainless steel components. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention provides a low-allergenic stainless steel, its preparation method, and its applications.

[0009] Specifically, the present invention is achieved through the following technical solutions: A method for preparing low-allergenic stainless steel, comprising: (1) Ferritic stainless steel billets were prepared by vacuum induction melting. (2) The steel billet is hot-rolled and cold-rolled in sequence to obtain rolled material; (3) Grind and degrease the surface of the rolled material, and then put it into a sealed atmosphere furnace for high-temperature nitriding; (4) After nitriding and heat preservation, the rolled material is immersed in ice water for quenching; (5) Remove the oxide scale from the surface of the rolled material to obtain low-allergenic stainless steel.

[0010] The above-mentioned method for preparing low-allergenic stainless steel, wherein the ferritic stainless steel billet comprises, by weight percentage: C: ≤0.0050%, Mn: <0.010%, P: ≤0.020%, S: ≤0.0020%, Cr: 18.0~20.0%, Mo: 3.0~4.0%, O <0.0015%, N <0.0080%, Nb: 0.35~0.65%, with the balance being Fe.

[0011] In the above-mentioned method for preparing low-allergenic stainless steel, the cold-rolled section shrinkage rate is >99%.

[0012] In the above-mentioned method for preparing low-allergenic stainless steel, the rolled material is wire or plate; wherein the diameter of the wire is ≤1.0mm; and the thickness of the plate is ≤1.0mm.

[0013] The above-mentioned method for preparing low-allergenic stainless steel results in a surface roughness Ra < 0.2 for the rolled material after grinding. m, then put it in acetone and degrease it using ultrasound.

[0014] The above-mentioned method for preparing low-allergenic stainless steel, wherein the high-temperature nitriding includes: evacuating the furnace cavity of the sealed atmosphere furnace to 0.1~0.5Pa, introducing nitrogen gas with a purity ≥99.99%, maintaining a constant pressure of 130~150kPa inside the furnace, a nitrogen gas flow rate of 3.5~4.5L / min, and heating to 1160~1180℃ and holding at that temperature for 1.6~1.8h.

[0015] In the above-mentioned method for preparing low-allergenic stainless steel, the cooling rate of the rolled material during quenching is ≥100℃ / s.

[0016] A low-allergenic stainless steel is prepared by the above-described preparation method.

[0017] The aforementioned low-allergenic stainless steel has a grain size of 70~90μm, a tensile strength ≥900MPa, an elongation ≥32%, and a work hardening coefficient ≥0.6GPa.

[0018] The aforementioned low-allergenic stainless steel is used in the preparation of medical stainless steel components.

[0019] The technical solution of the present invention has the following beneficial effects: (1) The low-allergenic stainless steel prepared by the present invention is superior to 316L stainless steel in terms of tensile strength and work hardening performance, and has excellent corrosion resistance and cell biocompatibility. It is suitable for mass production of various low-allergenic medical small metal components such as orthodontic wires and orthopedic tension band wires.

[0020] (2) The low-allergenic stainless steel prepared by the present invention is superior to or equivalent to commercial 316L medical stainless steel in cell compatibility test.

[0021] (3) The low-sensitivity stainless steel prepared by the present invention has a grain size that can be controlled at 70~90μm, a tensile strength of more than 900MPa, an elongation of more than 32%, a work hardening coefficient of more than 0.6GPa, which is higher than 0.5GPa of 316L, strong uniform plastic deformation ability, and resistance to alternating loads.

[0022] (4) The low-sensitivity stainless steel prepared by the present invention has no stress-induced martensitic phase transformation after nitriding, and has excellent tensile fracture toughness, which is beneficial to extending service life. Attached Figure Description

[0023] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention.

[0024] Figure 1 The X-ray diffraction pattern of a 1 mm diameter Fe-18Cr-3Mo ferritic stainless steel wire after nitriding for 1.6 h is shown, indicating that it has a fully austenitic structure. Figure 2 The austenite grain size is shown as approximately 80 μm after nitriding for 1.6 h on a 1 mm diameter Fe-18Cr-3Mo ferritic stainless steel wire. m. Detailed Implementation

[0025] To fully understand the purpose, features, and effects of this invention, the following detailed embodiments are provided. Except as described below, the process methods of this invention employ conventional methods or apparatus in the art. Unless otherwise specified, the terms and expressions used below have the meanings commonly understood by those skilled in the art.

[0026] This application provides a novel process for preparing nickel-free stainless steel, overcoming the difficulties in forming existing Mn-N nickel-based austenitic stainless steels and the problems of grain coarsening, thin nitrided layers, and poor plasticity in conventional nitriding processes. Medical implant materials have extremely high requirements for the corrosion resistance of stainless steel; therefore, this application selects Fe-18Cr-3Mo ferritic stainless steel, which combines corrosion resistance and formability. The nitriding temperature reaches 1160~1180℃, with numerous grain boundaries acting as nitrogen diffusion channels, enabling full-section austenitization in a short time. This suppresses grain coarsening caused by prolonged high-temperature holding, resulting in high-strength, low-allergenic, and biocompatible nickel-free stainless steel.

[0027] This invention addresses the technical challenges of existing nickel-containing austenitic stainless steel for medical use, which poses a risk of nickel sensitization, and the difficulties in forming existing nickel-free austenitic stainless steel, as well as the tendency of conventional nitriding processes to lead to grain coarsening and decreased plasticity. It proposes a novel technical concept.

[0028] The core concept of this invention is to reverse and enhance the "negative effects" of large-scale cold rolling deformation—namely, the highly elongated ultrafine grain structure and numerous grain boundaries—transforming them into "fast channels" for ultra-high-speed nitrogen diffusion. Specifically, this invention uses Fe-Cr-Mo ferritic stainless steel as the base material, which possesses excellent resistance to chloride ion corrosion and easy formability but insufficient strength. First, extreme cold rolling deformation refines the grains of the base material to their limit and stores high-density grain boundaries. Then, gas nitriding is performed at high temperature, utilizing the pre-stored numerous grain boundaries as fast channels for nitrogen diffusion. This allows nitrogen atoms to penetrate the entire cross-section of the wire / plate in a very short time and complete the in-situ austenite transformation, thus cleverly decoupling the contradiction between "high-strength nitriding" and "grain coarsening suppression." Finally, ice-water quenching fixes the single ultrafine-grained austenite structure obtained at high temperature to room temperature, ultimately yielding a nickel-free, high-strength, high-toughness, excellent corrosion resistance, and biocompatible stainless steel material with work hardening capabilities superior to 316L. This invention achieves a leap from "surface modification" to "overall tissue regulation," providing a completely new path for the mass production of low-allergenic medical stainless steel.

[0029] Specifically, the present invention provides a method for preparing low-allergenic stainless steel, which can be summarized into four major steps: preparation of ferritic stainless steel wire, surface pretreatment, high-temperature nitrogen nitriding, and ice-water quenching. Specifically, it includes: (1) Ferritic stainless steel billets were prepared by vacuum induction melting.

[0030] In some preferred embodiments, the ferritic stainless steel billet comprises, by weight percentage: C: ≤0.0050%, Mn: <0.010%, P: ≤0.020%, S: ≤0.0020%, Cr: 18.0~20.0%, Mo: 3.0~4.0%, O <0.0015%, N <0.0080%, Nb: 0.35~0.65%, with the balance being Fe.

[0031] (2) The steel billet is hot rolled and cold rolled in sequence to obtain rolled material.

[0032] In some preferred embodiments, the cold-rolled section shrinkage rate is >99%.

[0033] This invention helps to introduce ultra-fine elongated ferrite grains and a large number of grain boundaries into the rolled material by controlling the deformation of cold rolling to >99%, thereby achieving grain refinement before nitriding and providing conditions for rapid nitrogen diffusion.

[0034] Optionally, the rolled material is wire or sheet.

[0035] In some preferred embodiments, the diameter of the wire is ≤1.0mm and the thickness of the plate is ≤1.0mm.

[0036] (3) Grind and degrease the surface of the rolled material, and then put it into a sealed atmosphere furnace for high-temperature nitriding.

[0037] In some preferred embodiments, the surface roughness Ra of the ground rolled material is <0.2. m, then put it in acetone and ultrasonically clean it until it is fully degreased.

[0038] In some preferred embodiments, the high-temperature nitriding includes: evacuating the furnace cavity of the sealed atmosphere furnace to 0.1~0.5Pa, introducing nitrogen gas with a purity ≥99.99%, maintaining a constant pressure of 130~150kPa inside the furnace, a nitrogen flow rate of 3.5~4.5L / min, and heating to 1160~1180℃ and holding at that temperature for 1.6~1.8h.

[0039] Existing literature shows that the nitrogen saturation solid solution content of Fe-18Cr-3Mo matrix is ​​about 0.9% at 1160~1180℃, which is far lower than the critical concentration of 1.8% required for the precipitation of Cr2N. Therefore, no brittle nitrides are formed during the nitriding process, and the plasticity and corrosion resistance of the material are preserved.

[0040] (4) After nitriding and heat preservation, the rolled material is immersed in ice water for quenching.

[0041] In some preferred embodiments, the rolled material is immediately removed after nitriding and heat preservation and immersed in ice water for rapid quenching, wherein the cooling rate is ≥100℃ / s.

[0042] This invention uses rapid ice-water quenching to fix a single austenitic structure obtained at high temperature to room temperature. This quenching process can effectively avoid the precipitation of ferrite phase and various precipitates during the cooling process, thereby ensuring that the finished product maintains a single austenitic structure at room temperature and that the grain size remains in the refined state after nitriding.

[0043] (5) Remove the oxide scale from the surface of the rolled material to obtain low-allergenic stainless steel.

[0044] In some preferred embodiments, the surface oxide scale is ground off after quenching, and the surface roughness is controlled to Ra 0.2~0.4. m, yielding nickel-free austenitic stainless steel wire.

[0045] The low-allergenic stainless steel prepared according to the above method of the present invention has a grain size of 70~90μm, a tensile strength ≥900MPa, an elongation ≥32%, and a work hardening coefficient ≥0.6GPa, which meets the requirements for use in medical stainless steel components.

[0046] Example The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments, unless otherwise specified, are performed according to conventional methods and conditions.

[0047] X-ray diffraction was used to evaluate the crystal structure of the nitrided wire; the mechanical properties and work hardening coefficient of the austenitized wire after nitriding were measured according to GB / T 228.1-2021 "Metallic materials - Tensile testing - Part 1: Room temperature test method"; and the risk of sensitization was determined according to GB / T16886.5-2017 "Biological evaluation of medical devices - Part 5: In vitro cytotoxicity test".

[0048] Example 1 Ferritic stainless steel billets were obtained by vacuum induction melting; by mass percentage, the billets contained C: 0.005%; Mn: 0.008%; P: 0.020%; S: 0.0010%; Cr: 18.55%; Mo: 3.5%; O: 0.0012%; N: 0.0075%; Nb: 0.45%; with the balance being Fe.

[0049] The steel billet is hot-rolled and cold-rolled; the cold-rolled section reduction rate is 99.9%, and a φ1.0mm wire rod is prepared.

[0050] The surface of the formed wire is polished to a roughness of 0.1. m, place in acetone and ultrasonically clean until fully degreased.

[0051] The wire was fed into a sealed atmosphere furnace. The furnace chamber was first evacuated to 0.3 Pa, and then high-purity nitrogen with a purity of ≥99.99% was introduced. The furnace pressure was kept constant at 135 kPa, the nitrogen flow rate was 4.0 L / min, and the temperature was raised to 1180℃ and kept constant for 1.8 h.

[0052] Immediately after nitriding and heat treatment, the workpiece is removed and immersed in ice water for rapid quenching at a cooling rate of 105℃ / s. After quenching, the surface oxide scale is ground off, and the surface roughness is reduced to Ra: 0.3. m, yielding nickel-free austenitic stainless steel wire.

[0053] Testing revealed that the entire cross-section of the wire exhibited a single austenitic microstructure, with no detectable Cr. x Type N precipitates (see N-type precipitates) Figure 1 ), grain size approximately 85 m( Figure 2 The tensile strength was 937 MPa, the elongation was 38%, and the work hardening coefficient was 0.65 GPa. In vitro cytotoxicity tests showed a relative cell growth rate (RGR) ≥ 100%, with no risk of cytotoxicity or sensitization.

[0054] Example 2 Ferritic stainless steel billets were obtained by vacuum induction melting; by mass percentage, the billets contained C: 0.005%; Mn: 0.008%; P: 0.020%; S: 0.0010%; Cr: 19.5%; Mo: 4.0%; O: 0.0014%; N: 0.0075%; Nb: 0.37%; with the balance being Fe.

[0055] The steel billet is hot-rolled and cold-rolled; the cold-rolled section reduction rate is 99.999%, and a wire rod with a diameter of φ0.8mm is prepared.

[0056] The surface of the formed wire is polished to a roughness of 0.1. m, place in acetone and ultrasonically clean until fully degreased.

[0057] The wire was fed into a sealed atmosphere furnace. The furnace chamber was first evacuated to 0.3 Pa, and then high-purity nitrogen with a purity of ≥99.99% was introduced. The furnace pressure was kept constant at 145 kPa, the nitrogen flow rate was 4.5 L / min, and the temperature was raised to 1160℃ and held at that temperature for 1.6 h.

[0058] Immediately after nitriding and heat treatment, the workpiece is removed and immersed in ice water for rapid quenching at a cooling rate of 115℃ / s. After quenching, the surface oxide scale is ground off, and the surface roughness is reduced to Ra: 0.3. m, yielding nickel-free austenitic stainless steel wire.

[0059] Testing revealed that the entire cross-section of the wire exhibits a single austenitic microstructure with a grain size of approximately 75. The cell diameter is m, tensile strength is 965 MPa, elongation is 36%, and work hardening coefficient is 0.63 GPa. In vitro cytotoxicity tests showed a relative cell growth rate (RGR) ≥ 100%, with no risk of cytotoxicity or sensitization.

[0060] Comparative Example 1 (Commercial 316L Stainless Steel) The φ0.8mm standard solution-treated 316L (S31603, 022Cr17Ni12Mo2) wire produced according to GB / T 4240-2019 "Stainless Steel Wire" has a tensile strength of approximately 685MPa, a work hardening coefficient of 0.5GPa, and contains 13.4wt% nickel. In vitro cytotoxicity tests show that the relative cell growth rate (RGR) is 85-96%, indicating slight toxicity and a risk of sensitization.

[0061] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0062] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0063] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A method for preparing low-allergenic stainless steel, characterized in that, include: (1) Ferritic stainless steel billets were prepared by vacuum induction melting. (2) The steel billet is hot-rolled and cold-rolled in sequence to obtain rolled material; (3) Grind and degrease the surface of the rolled material, and then put it into a sealed atmosphere furnace for high-temperature nitriding; (4) After nitriding and heat preservation, the rolled material is immersed in ice water for quenching; (5) Remove the oxide scale from the surface of the rolled material to obtain low-allergenic stainless steel.

2. The method for preparing low-allergenic stainless steel according to claim 1, characterized in that, The ferritic stainless steel billet, by weight percentage, comprises: C: ≤0.0050%, Mn: <0.010%, P: ≤0.020%, S: ≤0.0020%, Cr: 18.0~20.0%, Mo: 3.0~4.0%, O <0.0015%, N <0.0080%, Nb: 0.35~0.65%, with the balance being Fe.

3. The method for preparing low-allergenic stainless steel according to claim 1, characterized in that, The cold-rolled section shrinkage rate is >99%.

4. The method for preparing low-allergenic stainless steel according to claim 1, characterized in that, The rolled material is either wire or sheet; wherein the diameter of the wire is ≤1.0mm; and the thickness of the sheet is ≤1.0mm.

5. The method for preparing low-allergenic stainless steel according to claim 1, characterized in that, The surface roughness Ra of the polished rolled material is less than 0.

2. m, then put it in acetone and defatted by ultrasound.

6. The method for preparing low-allergenic stainless steel according to claim 1, characterized in that, The high-temperature nitriding process includes: evacuating the furnace cavity of the sealed atmosphere furnace to 0.1~0.5Pa, introducing nitrogen gas with a purity ≥99.99%, maintaining a constant pressure of 130~150kPa inside the furnace, a nitrogen flow rate of 3.5~4.5L / min, and heating to 1160~1180℃ and holding at that temperature for 1.6~1.8h.

7. The method for preparing low-allergenic stainless steel according to claim 1, characterized in that, During quenching, the cooling rate of the rolled material is ≥100℃ / s.

8. A low-allergenic stainless steel, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 7.

9. The low-allergenic stainless steel according to claim 8, characterized in that, The low-sensitivity stainless steel has a grain size of 70~90μm, a tensile strength ≥900MPa, an elongation ≥32%, and a work hardening coefficient ≥0.6GPa.

10. The use of the low-allergenic stainless steel according to claim 8 or 9 in the preparation of medical stainless steel components.

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